Start FBX animation with Tpose
Be sure to be in Pose mode with your armature selected.

In the Dope Sheet Editor, select all the keys (A) and move them some frames to the right. (G25Enter for 25 frames = 1 second)

Go to frame 1, in the 3D View select all the bones, press Alt+R and Alt+G to reinitialize position and location. The rig should be in T pose now.

Select all the bones, in the 3D View press i to insert a key on location and rotation (LocRot).